Abstract
Conventional infrared thermography techniques, including pulsed and lock-in thermography, have shown great potential for non-destructive evaluation of broad spectrum of materials, spanning from metals to polymers to biological tissues. However, performance of these techniques is often limited due to the diffuse nature of thermal wave fields, resulting in an inherent compromise between inspection depth and depth resolution. Recently, matched-filter thermography has been introduced as a means for overcoming this classic limitation to enable depth-resolved subsurface thermal imaging and improving axial/depth resolution. This paper reviews the basic principles and experimental results of matched-filter thermography: first, mathematical and signal processing concepts related to matched-fileting and pulse compression are discussed. Next, theoretical modeling of thermal-wave responses to matched-filter thermography using two categories of pulse compression techniques (linear frequency modulation and binary phase coding) are reviewed. Key experimental results from literature demonstrating the maintenance of axial resolution while inspecting deep into opaque and turbid media are also presented and discussed. Finally, the concept of thermal coherence tomography for deconvolution of thermal responses of axially superposed sources and creation of depth-selective images in a diffusion-wave field is reviewed.
Highlights
Active thermography is a rapidly growing non-destructive testing (NDT) technique, which, since the 1980s [1], has been widely employed in research settings for detection of defects in a broad range of materials
The principle behind active thermography is quite straightforward: the sample is exposed to some form of excitation to create a thermal-wave field inside sample while the surface temperature is constantly registered by an infrared camera
Incorporation of matched-filtering in thermography has been proposed [21,22,23,24,25] for overcoming the classic compromise of conventional active thermography in order to maintain depth resolution while inspecting deep in samples
Summary
Active thermography is a rapidly growing non-destructive testing (NDT) technique, which, since the 1980s [1], has been widely employed in research settings for detection of defects in a broad range of materials. Both techniques are popular for inspection of industrial samples. They have been widely used for detection of damage in Carbon Fiber Reinforced Plastic (CFRP) materials [2], inspection of airplane parts [3] and detection of leakages in integrated circuits [4]. Depending on the application, different types of external excitation, such as optical [7], magnetic [17], mechanical waves [19], electrical [4,16] or even cyclic stress/strain [18], can be utilized to induce the thermal wave field inside the sample. A comprehensive review of conventional thermography techniques such as PT and LIT can be found in another paper of this special issue on “Novel Ideas for Infrared Thermography Applied in Integrated Approaches”
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